Distillation Column Control Design

8/20/2019 Distillation Column Control Design

1/22

Strategy

for

Distillation-Column

Control

i chemical plants and petroleum refineries, there

are, today,

many distillation

columns that

are

working well. There are also many others tha t are not working

well,

and

at least a few that function very poorly, or not a t all.

Failure

to obtain

performance specified by the column design engineer is due, in many cases,

to

faulty or inadequate control system design. Troubleshooting of columns that

are already in operation is frequently necessary, but practical considerations

usually

limit

corrective measures to relatively minor items. Proper original

design is by far the best way to guarantee satisfactory operation and control.

Therefore, in this book we will approach the design of integrated distillation-

column control systems as a systems problem in process design. The application

of feedforward, feedback, and protective controls wdl be coordinated with the

sizing and proper location of process holdups

to

achieve both automatic start-

up and shutdown and smooth, noninteracting control of column product

compositions.

1

.i DISTILLATION CONTROL OBJECTIVES

The starring point of any design project is a definition of objectives. For

distillation there are many possible approaches, but the one chosen here is one

the authors have found broadly

useful

in virtually

all

kinds of processes.’ It

has three main facets: 1)material-balance control, (2) product quality control,

and

3)

satisfaction of constraints.

As

applied specifically

to

distillation columns,

this philosophy suggests the following:

1. Material-balance control”

This term is sometimes used by others”

to

mean a control system in which reflux is

set

by reflux drum level control, and distillatelfeedratio is set manually

o r

by a com position (temperature)

controller. The authors of this book have been unable to find any special merit for this scheme

except for some high reflux ratio

columns.


2/22

4 StrMeD

J -

Dthdktim-Column Control

-The column control system must

cause

the average

sum

of the product

streams

to be

exactly equal

to

the average feed rate.

Harbed4

has called

this requirement that of keeping the column

in

“balance.”

-The resulting adjustments in process flows must

be

smooth and gradual

to

avoid upsetting either the column or downstream process equipment

fed by the column.

--Column holdup and overhead and

bottoms

inventories should

be

maintained between maximum and

minimum

l i mts

It is important

to

note that the material-balance controls on any

given column must be consistent with the material-balance controls on

adjacent process equipment. In most cases material balance will

be

con-

trolled by so-called “averaging” liquid-level or pressure controls.

2.

Product quality control

The control system for a binary distillation in most cases must:

-Maintain the concentration of one component

in

either the overhead

or bottoms at a specified value.

-Maintain the composition at the other end of the column as close as

possible

to

a desired composition.

t

It

is

usually true

that

minimum

operating cost

is

achieved when the

products are controlled at

minimum

acceptable purities.

This is

so

because

the

relationship between thermodynamic work of separation and purity

is nonlinear.” For some columns compositions

are

allowed

to

vary at

one end, and sometimes both ends, to satisfjr cert in economic constraints.

Both material-balance and composition controls must function sat-

isfactorily

in the

face

of

possible disturbances

in:

-Feed flow rate

-Feed composition

-Feed thermal condition

-Steam supply pressure

--Coohg-water supply temperature

--Cooling-water header pressure

-Ambient temperature, such

as

that caused by rainstorms

3. Satisfaction of constraints

For

safe,

satisfactory operation of the column, certain constraints

must be observed. For example:

-The column shall not

flood.

--Column pressure drop should be high enough to maintain effective

column operation, that

is

to

prevent serious

weeping

or dumping.

t

For

multicomponent columns subjected

to

feed composition changes, it is not

possible

to

hold exactly constant the compositions

at

both ends of the column;

the

composition at one end

must change

a little.

If feed variations are in the nonkey components, composition may

vary

somewhat at

both

ends of

the

column.With only two drawoffs, we can conml two keys, but

not the nonkey components.


3/22

1

I

Distillutk

Control O&jem’ves

5

-The temperature difference in the reboiler should not exceed the

critical temperature difference.

--Column feed rate should not be

so

high as to overload reboiler or

condenser heat-transfer capacity.

-Boilup should not be

so

high that an increase will cause

a

decrease

in product purity at the top of the column.

--Column pressure should not exceed a maximum permissible value.

There are, in addition, several other facets of column control.

Startup and Shutdown

Column controls should facilitate startup and shutdown and, by implication,

Transitions

When it is desired to change product compositions, the column controls

should facilitate doing

so.

This is particularly important when feed stock com-

position varies widely and it is desired to optimize column o r train operation,

as, for example, with a computer.

Heat Recovery

Increasingly there is an interest in recovering as much heat as possible. The

petroleum industry has frequently used the sensible heat in a column bottom

product to preheat column feed. Recently more ambitious schemes have been

employed in which the reboiler for one column is used as tha condenser for

another. Such schemes magnify control problems and sometimes limit process

turndown.

Testing

There should be enough instrumentation so that testing may be carried out

Miscellaneous

In addition to the above, column control should be designed with human

engineering in mind. For example:

-The operator‘s work station, whether a cathode ray tubekeyboard console

or a panelboard with gages, switches, recorders, and so on, should be carefully

designed according to human engineering principles for easy

use.

-The controls should be

so

designed as to require minimum maintenance.

The need for frequent or critical “tuning” should be avoided. The hardware

should be designed and arranged for convenient access and quick repair or

replacement.

-The control system design engineer should use the simplest possible

design procedures, not only to hold design costs down, but also so that the

work can be readily discussed with other design and plant personnel. This will

facilitate, at a later date, any necessary minor redesign a t the plant site. Failure

should make it easy to achieve total reflux operation when desired.

for tray efficiency, heat and material balances, flooding, and so on.


4/22

6 tratemfw

Didhttim-Column

Control

of design and plant personnel

to

achieve a mutual understanding of design

objectives, concepts, and methods is one of the frequent causes of unsatisfactory

plant operation.

1.2 ARRANGEMENTS FOR MATERIAL-BALANCE CONTROL

As

shown elsewhere,’ the size and location of

tanks

and the concept of

overall material-balance control used

can

have a great influence on plant investment

and process control. If the design engineer

uses

the concept of

control

in

the

directwn opposite t o w tanks

may

be

smaller and plant fned investment and

working capital can be lower than if

umtroZ in the directwn

es used. The

meaning of these expressions is illustrated, for simple tanks with level controllers,

in

Figures 1.1 and

1.2.

For the last tank in a series (final product storage), the demand flow is

always the shipments to a customer. As shown by Figures 1 . 3 and 1.4, the

choice in control strategy is between adjusting the flow into the last tank (control

in direction opposite to flow) or adjusting flow into the first process step

(control

in

direction of flow). In the first case, we can easily

use

simple automatic

controls.

In

the second case,

it

is more common to have an operator make the

adjustment.

When more than one tank

is

involved, other advantages of control in the

direction opposite

to

flow

are

(1)

less difficulty with stability problems, and

(2) reduced internal turndown requirements. “Turndown,”

as

used here, is the

ratio of maximum required flow rare to minimum required flow rate. In this

instance the meaning is that, in response

to

a given change in demand flow,

the required change in the manipulated flows will be smaller in one case than

in the other.

Once

the

basic concept of material-balance control

has

been selected for a

process, one must apply the same concept to

all

process steps. It is for this

reason

t h a t

the first step in designing column controls is

to

determine the

material-balance control arrangement. Control in the direction of flow is the

most commonly used concept (although the least desirable), and a frequently

encountered arrangement is shown on Figure 1.5. Here level in the condensate

receiver (also commonly called reflux

dr um

or accumulator)

sets

the top product,

or distillate flow, while the level in the base of the column sets the bottom

product flow; in other columns base level sets steam or other heat-transfer

media

to

the reboiler, in which case the condensate receiver level

sets top

product flow.

Generally speaking the direction of material-balance control is determined

by the demand stream. In recycle systems we may find some material-balance

controls in the direction of flow while others are in the direction opposite

to

Material-balance control, in the direction opposite to flow, can lead

to

many

interesting level-control and flow-ratio options.

These are

discussed in derail

in Chapter 6 .


5/22

1 2 A w a q m ~

m

M a t d - B a l a n c e

Control 7

FIGURE 1.1

Material balance control in direction opposite

to

flow

FIGURE 1 2

Material

balance

control in direction of

flow


6/22

3

G

c

Q

c

sa

a

c a

U

C

E

L

2

m

k

@

?

E

a

3

2

c

C

sB

W

E


7/22

C

Q

3

L

3

C

8

2

8

z

E

3

m

C

I

-

c

.

3

8

8

-

c

m

m

-

?

E

z

=

3

o

i

U


8/22

10

Strategy for

Dljtillatwn-Column Control

FIGURE 1.5

Distillation column with material balance control in direction

of

flow


9/22

1.3

Funahmentnls ofComposition Control 11

1.3

F U N D M E N T L S O F C O M P O S IT IO N C O N TR O L

Let us consider briefly what must be done to

a

column to keep terminal

compositions constant on

a

steady-state basis when the column is subjected

to

sustained changes in feed flow rate or feed composition. Methods of handling

other disturbances will be discussed later.

To simpl@ the analysis, let us limit

our

attention to an ideal, binary distillation.

This is somewhat restrictive, although the results will be applicable in

a

general

way

to

multicomponent systems, particularly those that may be treated as quasi-

b&ary or pseudobinary systems.

As will

be

shown in Chapter

2,

if feed composition and feed thermal

condition are constant, then we want the “operating lines” on

a

McCabe-

Thiele diagram to remain constant when feed flow changes. The operating lines

(defined in the next chapter) will not change as long as the distillate-to-feed,

reflux-to feed, boilup-to-feed, and bottom-product-to-feed ratios

are

held constant.

Practically speaking one may hold

all

four ratios constant by fixing anv one of

the three pairs: (1) the reflux-to-feed ratio and

the

boilup-to-feed

ratio,

(2)

the reflux-to-feed ratio and

the

bottom-product-to-feed ratio, and

(3)

the boilup-to-feed ratio and the distillate-to-feed ratio. In considering case

( 1 ) ,

for example, we see tha t if the rectification section vapor-to-feed ratio is

fixed (and it will be if the boilup-to-feed ratio is fixed) and the reflux-to-feed

ratio is fixed, then the distillate-to-feed ratio will be fixed, since the distillate

flow is the difference between the reflux flow

and

the vapor flow. Similarly,

the bottom-product-to-feed ratio will be fixed, since the bottom product flow

is the difference between

the

stripping section liquid and the boilup.

If feed rate and feed composition are not constant, Rippin and Lamb’ have

shown that, for small perturbations, one should change the boilup and reflux

according to the following equations:

A V = K ~ A z F

KfzAF

ALR

=

Kf3AzF

Kf4A.F

where

V = vapor flow from the reboiler

LR = internal reflux flow at

the

top of the column

The A’s represent departures from average operating conditions. The constants

KO

Kf4

may be calculated approximately by the column designer. Luyben6

has shown that it is necessary to be quite careful in designing feedforward

compensation for feed composition changes, particularly when the column is

not making a sharp separation.

Visualization of column operation,

in

terms of reflux-to-feed and boilup-

to-feed ratios, was suggested by Uitti7 and has since been proposed in varying

degrees bv many others. Throughout the rest of this book, it will be used as

the primar). basis for column composition control.


10/22

12

StrateBy

fmDirtillatMn Column Control

It should be noted, however, that this feedforward approach

to

column

control has a particular limitation: In general one cannot calculate the constants

Kfl -.

Kf4

with great accuracy. For columns that are not operating

too

close

to either upper dr lower h i t s of capacity, small changes in feed rate, and

consequent changes in boilup and reflux, will not change tray efficiency appreciably.

The terms Kf nd

Kf4

therefore will be constants. If the feed composition

changes are not too large (as will usually be the case), then Kflnd Kf3 ay

also be treated as constants. To determine the control accuracy obtainable by

this approach, one should make the necessary calculations or tests for each

individual column. Where really close control is required, one must supplement

feedforward control with measurement of the column terminal compositions

and subsequent feedback control,

a t

least

a t

one end of the column. The usual

philosophy will be to

use

feedforward for fast, approximate control and feedback

for long-term, accurate control of composition.

It should be noted,

too,

that feedforward from feed composition may not

be needed if the feed comes from

a

process step with discharge composition

control. Feedfonvard compensation for other process variables, such as bottom

product or distillate demand flow, is discussed in Chapter

6.

As will be shown later (Chapters 5 and 20), a properly designed column

feed system can play

a

very important role in filtering out hsturbances in feed

rate, feed composition, and feed enthalpy, thereby making composition control

much easier.

1.4

COMPENSATION FOR VARIOUS DISTURBANCES

Feed Thermal Condition

Feed should enter the column with a constant enthalpy. When significant

changes are anticipated, a heat exchanger and feed-enthalpy control system

should be provided. This is discussed in more detail in Chapters

5

and

11.

Steam

Supply

Pressure

Changes here can cause changes in boiling rate. The best preventive is a

steam-to-feed ratio control system combined with temperature and pressure

compensation of steam flow. A high pressure drop across the steam valve favors

smooth control but velocity-limiting trim may be required

to

minimize noise

and plug and seat wear. A high pressure drop is also undesirable for energy

conservation. If the pressure drop is high enough, sonic flow through

the

valve

results, and reboiler steam-side pressure has no effect on flow rate. The design

should avoid having sonic flow corresponding to low feed rates and nonsonic

flow corresponding

to

high feed rates, since required controller gain changes

make tuning v ry difficult.


11/22

1.5

Startup and Shutdown 13

Cooling-Water Supply Temperature

Cmling-water temperature

changes

are

usually

seasonal, and will require

n o specific correction.

If, for

some reason,

they are

large

and rapid, then it

may

be

desirable

to

provide an enthalpy control system for the condenser.

By

measuring the temperature rise of the cooling water across the condenser, and

multiplying

i t

by

the

cooling-water flow rate, one has a measure of the heat

transferred, 4.. This calculated value of qc can

serve

as the measured variable

in an enthalpy control system. For column pressure control, the enthalpy control

system can serve as the secondary loop in a cascade system.

Cooling-Water Header Pressure

One

of

the best ways

to

make a flow system immune

to

pressure changes

is

to

provide a high system pressure drop. This, however, is costlv. Another

way, which should be satisfactory for some distillation columns, is to

use

a

cooling-water flow control system. It does, however, have limitations, which

will be discussed later.

Ambient Temperature

If the column, auxiliaries, and piping are properly insulated, and if the

column is properly controlled, ambient changes should cause little difficulty

unless

the condenser is of

the

air-cooled type. In this event it may be desirable

to

use an internal reflux computer, discussed in Chapter 11. If, as is often the

case, the vapor piping from the top of the column to the condenser is both

uninsulated and long, ambient temperature changes mav cause fluctuations in

pressure and the rate of condensation.

1.5

STARTUP AND SHUTDOWN

Startup and shutdown are often dismissed as relatively unimportant, since

they happen

so

seldom that

i t

is not economical to spend much time and monev

on improvements. This may be true in a petroleum refinery where shutdowns

may occur at intervals of two or three years. In the chemical industry, however,

where process equipment is often plagued by severe corrosion or by plugging

process materials, startups and shutdowns are far more common- perhaps

monthly, weekly, or even daily. Further, elaborate heat and material recycle

schemes may require inmcate startup/shutdown procedures as part of the original

design. To put it another way, columns and their control systems may have

to be designed specifically

to

accommodate

a

particular startup/shutdown

procedure.

Columns are commonly started up in

total

reflux-no product is taken off

at top or bottom. Limited experience, however, suggests that faster and smoother


12/22

14

Stratgy for Dtddhtkfi-Cul~mnuntrui

startups may be achieved by recycling top and bottom products back to feed

during part of the startup sequence.

Startup/shutdown will be discussed in more detail in Chapter 9.

1.6

CONTROL SYSTEM DESIGN PHILOSOPHY

Current

Design

Practices

In

the

preface we noted that we would try, in this book,

to

present

a

multivariable control approach to distillation column control. Before discussing

this, however, let

US

look a t typical controls in existing plants.

There is a traditional pattern of what is called “instrumentation” in the

chemical and petroleum industries based on single-loop control (sometimes

called SISO-single input, single output). Each process operation has a number

of independent or single loops for feedback control of temperatures, pressures,

flows, liquid levels, and sometimes compositions. The term “single loop” means

there is one measurement, one controller, and one final control element, usually

a

valve. Many, but not all, of these loops are represented in the central control

room (CCR)

by control stations.

The controllers usually have neither antireset windup nor automatic tracking,

and there is little or no logic circuitry

to

tie the many loops together. This

statement is true for both analog

and

some digital hardware. As a consequence

the operators must perform startup operations with the control stations switched

to “manual,” and must implement process logic by switching in and out of

automatic.” Since the original design procedures are usually qualitative and

intuitive, with heavy emphasis on conformity to past practices, it is not surprising

that some loops never work in “automatic.” Others, although stable in “automatic,”

are

so

sluggish that they are ineffective in dealing with typical ‘disturbances.

For newer plants with higher throughputs, operation closer to hard constraints,

smaller and fewer holdups, energy-recovery systems, and elaborate material-

recycle systems, the traditional “instrumentation” approach to control is often

seriously inadequate.

For some years now, progress in single-loop design has been essentially at

a standstill.

As

recently as 1950, process control was hardware limited. Since

then primary measuring elements, controllers, computing devices, control stations,

and control valves have improved greatly in reliability, sensitivity, and speed

of response. Consequently these characteristics

less

and less frequently pose

limitations

to

the single-loop designer. Further,

the

quality

of

control achieved

by single-loop systems is not greatly affected by type of hardware; it makes

little difference, for a given algorithm, whether one uses analog pneumatic or

electronic gear, microprocessor controls, or

a

digital computer.* In addition,

Digital computers and microprocessor controls, however, usually offer a wider range of

controller parameter adjustments and facilitate the design of control systems more sophisticated

than most of those discussed in this

book.


13/22

1 6

Control System Des

Philosop@

15

optimized tuning procedures for unaided feedback controllers have limited

practical value for continuous processes; they yield results that are far inferior

to

those obtainable with well-damped feedback controls with simple feedforward

and override control.*

For plants being designed today (late 1983), it is increasingly common

to

use

microprocessor controls instead of analog

see

discussion under “Hardware

Conventions and Considerations”).

Multivariable Control

To avoid the limitations of single-loop design and to provide a more flexible

and sophisticated process operating logic than can be implemented by human

operators, we use an approach we call multivariable control.’ Many definitions

of this term may be found in the literature, but most of them are expressed

in mathematical terms rather than

in

terms of process hctions. For our purposes

we define a multivariable control svstem as one that has the built-in intelligence

to look simultaneously at two or more process variables and to choose, in a

given situation, the best of several preprogrammed strategies (algorithms) for

manipulating one or more control valves (or other final control elements).

For example, the steam valve for a distillation column reboiler, depending

on circumstances, may respond to controllers for:

Steam flow rate

Column

AP

Column pressure

Base temperature

Column feed rate

Column base level

Column bottom-product rate

The seven variables listed may also

exert

control on five or

six

other valves.

To

provide automatic control of this sort, we make extensive use of “variable

configuration” controls that

are

usually implemented by overrides.

I f

for instance,

base composition is normally controlled by steam flow that can be taken over

or overriden by high column AP, this is a variable configuration. If base level

control is normally achieved bv a PI controller that can be overridden by high

or low base level proportion&-only controls, we call that “variable structure.”

Multivariable control may involve both variable configuration and variable

structure

controls. Hardware permits us

to

automate this kind of control with a speed,

precision, and reliabilinr that are completely beyond the capabilities of human

operators.

It is common to think of process control fimctions stacked one above another

in

a pyramid or hierarchic arrangement as with traditional business or military

organization structures, particularly when computers are involved. Multivariable

control structures, however, with their extensive lateral and diagonal crossovers,

are functionally more like the modern “matrix” concept of management. From


14/22

16 Strategy

fm

Distillation-Column

Control

a control engineering standpoint, the shorter lines of communication

and

de-

centralized control functions permit more rapid and stable control, and more

reliable, troublefree operation.

By now it is probably apparent tha t we are striving for control system

designs whose performance and design parameters are specified in advance of

plant startup. In practice we furnish calibration data for controller parameters

and computational devices for the majority of control loops prior

to

startup.

We calculate these from simulations or simple linear models. For microprocessor

computer controls, we calculate scaling parameters for computation blocks

(either in software or hardware). Our design procedures are accurate enough

that only a modest amount of empirical controller tuning is required at startup.

Stability Speed

of

Response, and Interactions

Most existing literature on automatic control is concerned with the stability

and speed of response of single loops. The traditional objectives of feedback

control system design are:

1.

To

get

the

fastest possible response to set point changes.

2.

To

compensate for or

to

attenuate disturbances as much and as quickly

as possible. These must be accomplished with a reasonable degree of closed-

loop stability.”

In process control the objectives are often quite different. The objectives

of averaging level control, for example, clearly are different from those just

mentioned. A typical chemical

plant

o r refinery has hundreds of single loops

with many interactions among them. It is usually far more important to design

for

a

dynamic balance among these loops with

a

minimum of interaction than

to

strive for maximum speed of response. Further, it

is

usually undesirable

to

make rapid changes in manipulated variables since these may upset the process.

For example, distillation column reflux flow and boilup should not be changed

too rapidly since these might cause transient flooding or weeping in part of

the column. Our preferred philosophy of controller tuning is discussed in a

book’ and a paper.”

There are five simple methods by which to make a system noninteracting:

1. Design material-balance control loops

to

be at least a factor of 10 slower

than related composition control loops. Similarly, in cascade systems, make the

secondary or slave loop at least a factor of

10

faster than the master loop.

2

Avoid designs

that

are intrinsically interacting,

such

as pressure control

and

flow control at the same point in a pipeline. One of the two controllers

must be detuned.

3.

Select process designs that eliminate or minimize interaction. For example,

Some processes are open-loop unstable,

which

means that controls must be added and

must

be

kept in “automatic” for stable operation.


15/22

1 6 ontrol

System

Des@

Philosqphy

17

if a tubular reactor is fed at several points along its length from a common

header, flow-rate interactions may be reduced to an arbitrary level by providing

a very high pressure drop across each feed valve in comparison with the reactor

pressure drop.

4

Use

override circuits. Although not specifically intended for this purpose,

override circuits, by permitting only one controller at

a

time to control a given

valve, eliminate interactions.

5. Use interaction compensators (decouplers). If two control loops, such

as top and bottom temperature controls on

a

distillation column, interact, we

can eliminate the interactions by installing

two

compensators. One compensates

for the action of the top temperature controller on the bottom loop while the

other compensates for the action of the bottom temperature control loop on

the top one. This is discussed in Chapter 20.

In

addition, there are some very sophisticated mathematical methods for

dealing with

interaction^.^^,^^ ^^

Some are intended for noninteracting design

while others seek a design that provides an optimum amount of interaction.

Hardware Conventions and Considerations*

For control loops represented in

the

CCR (central control room), it is

normal practice (as mentioned earlier)

to

furnish “control stations.” These may

be analog pneumatic, analog electronic, or microprocessor based. In the last

case, the station may be physically distinct, like an analog station, or may be

represented on

a

CRT display as a “faceplate.” Each provides an indication

of

the process variable (flows, level, temperature, etc.), the desired value (set point),

and the valve loading signal (controller output, really). There is also a “manual-

automatic” switch, which some vendors label “hand-automatic.” In the “manual”

mode, the feedback controller is disconnected and there is a knob that enables

the operator remotely

to

set the valve position. This may or may not be subject

to

restrictions imposed bv feedforward compensation, overrides, and

so

on,

depending on the design philosophy for a particular project.

If

cascade control is involved, as, for example, liquid level control cascaded

to flow control, the secondary station not only has manual-automatic switching,

but

also

another fUnction-“remote-locaI.”

In

the “remote” position, the secondary

controller set point comes from the output of the primary controller. In the

c‘loca”’ position, valve position may be set manually or the controller set point

may be set by the operator (c‘local-auto”).Although cascade functions are

sometimes combined into one station (or CRT “faceplate”) for space and

money-saving reasons, we recommend dual stations. Most single-station designs

with which we are familiar are very inflexible and complicated; they do not

permit ready implementation of feedforward, overrides, and

so

forth.

Much

of this

section may seem superfluous

to

instrument and

plant

personnel.Our experience

however with process design engineers column designers and so

for& is

that this discussion

may be very helpfid.


16/22

18 Strategyfm Dirtillation-Column Control

As of this writing (late 1983),the two biggest hardware needs appear to

1.

Better measurements, especially of compositions.

2.

Better control valves. With regard to these, more progress has been made

in

the design of valve bodies and trim than in the design of actuators. Valve

positioners should always be used (it is assumed in this book that they will

be)

. Piston actuators with double-acting, two-stage positionersare recommended.

As far as CCR hardware is concerned, we have

a

decided preference for

microprocessor controls. They

are

technically more versatile and

are

less expensive

(some versions) than analog. As of late 1983, many are featuring satisfactory

antireset windup and override capabilities. In adchion, they provide more

advanced logic capability, dead-time simulation, and adaptive tuning. Some of

the last named achieve self-tuning via stochastic techniques or by pattern rec-

ognition. Others have gain scheduling, where reset time and proportional gain

are functions of some process variable or the controller error signal.

Microprocessor controls usually have a sampling time of a fraction of a

second. Although slightly slower than analog controls, their performance can

generdy be approximated by analog control algorithms.

Other advantages include freedom from drift, and the fact that they can be

calibrated more precisely, can be reconfigured or restructured without wiring

changes, have a larger range of tuning parameters, and contain more control

algorithms.

For most projects today, it is possible

to

find worthwhile applications for

a

supervisory digital computer with

a

good data historian, regardless of the

type

of basic controls selected (pneumatic, electronic, or microprocessor). Digital

readouts for important variables are worthwhile because they permit seeing

their magnitudes with sensitivity approaching that of the original analog mea-

surements. Most typical analog measurements have a sensitivity ranging from

one part per 1000 to one part per

10,000.

Most analog readout devices,

however, are limited to

0.5-2

percent.

For maximum advantage a supervisory computer should be programmed

to have the control algorithms discussed in Chapter 12. These are position

rather than velocity algorithms.

It

is our opinion that using such

a

computer

to imitate unenhanced two nd three-mode analog controls is poor practice.

Some worthwhile applications for computers will be discussed later.

Computer consoles were originally provided in the

CCR

for the convenience

of

the

operators. Sometimesa consolidated console is also provided for production

supervision. Engineers’ consoles, perhaps

at

another location, facilitate technical

studies. Separate consoles for maintenance personnel

(the

computer can be a

powerful maintenance tool) are highly desirable.

Most control valves today are either single-seated global types

or

rotary types.

For both

there are substantial, coercive stem forces when the valves are in flowing streams. Positioners

compensate

for

this and maintain the valve’s inherent flow characteristic expressed as a function

of

controller output signal.

It

should be noted, however, that some users prefer not

to

use

positioners.

be:


17/22

1.8 Column Des@ Philosophy

and

Control System Des@ 19

PROCEDURE FOR OVERALL CONTROL SYSTEM DESIGN

We now have enough information to suggest a sequence of design steps

to follow that will lead to a quantitative definition of column and hardware

performance.

1. After careful discussion with the process engineer and column designer,

and after careful review of the overall process flow sheet, prepare a simplified

flow sheet that defines the control concepts:

a. Select the overall material-balance control scheme first, preferably

proceeding from final product inventory

to

raw material inventory.

ll

individual equipment-piece material-balance controls must be consistent

with this scheme.

b. Select composition control schemes.

c. Add feedforward and interaction compensators as required.

d. Add protective controls and automatic startup/shutdown circuits.

e .

Add miscellaneous temperature and pressure controls.

2. Prepare material-balance and composition-control signal flow diagrams.

3. Determine holdup volumes required for smooth material-balance control

4.With holdups determined calculate column-composition transfer

functions.

5. Select measurement spans and calculate control valve sizes.

6 .

Calculate feedforward and interaction compensators.

7. Calculate

all

other overrides.

8. Calculate feedback-controller gain and reset settings and control-loop

natural frequencies. Check feed-tank material balance and mixing time constants

for adequacy.

9. Use simulation for some columns, particularly those in critical service

or with a new untested control system

o r

process configuration. Simulation of

the column and its control system will be usefd in confirming control concepts

and controller tuning parameters.

It

may also save startup time.

Hardware vendors may now be selected and the measurement and control

equipment may be ordered.

Part

I1

of this book deals with the qualitative and heuristic aspects of steps

1 and

3.

Quantitative information for the other steps is presented in Part 111.

1 7

and for liquid-level override controls at each end of the column.

1.8

COLUMN DESIGN

PHILOSOPHY

AND CONTROL SYSTEM DESIGN

Experience on many projects shows that even small, simple columns benefit

from a modest application of feedforward compensation and overrides. This is

due to the trend toward increasinglv tight column design. The number of trays

is held down because smaller allowan&

are

made for uncertainty in tray efficiency.

Column diameter and tray spacing are now kept

to

smaller values, with the


18/22

2 StrategyfmDljtillatwn-Column Control

result that columns typically operate closer to flooding. The combined effect

of these design policies is to make columns much touchier and harder to control.

Experience indicates that typical incremental instrument investment over

that required for unenhanced feedback controls is

5-10

percent (large projects

tend toward the lower figure). This incremental investment not only provides

better normal control, but it also helps

to

avoid inadvertent shutdowns.

It

is,

therefore, our opinion that these controls should be used

to

some extent on

almost every column.

Suppose, however, that the customer insists on minimum application of

feedback controls with

no

feedfonvard compensation or overrides. What column

design philosophy should be followed?Having had considerable adverse experience

with columns with primitive controls, particularly sidestream drawoff columns,

and columns with heat recovery schemes, we suggest the following:

1. Design for normal operation

a t

60 percent of the rate for flooding.*

2.

Provide five extra trays or increase the number of trays by

10

percent,

3.

Provide increased tray spacing.

4.

Provide larger condensers. If water cooled,

use

tempered water.

5.

Provide larger reboilers (lower heat flux).

6.

Control column

Al’

by boilup, or

flow

control steam.

7.

Provide

100

percent reserve capacity in heat-recovery schemes or avoid

8.

Avoid sidestream drawoff designs.

9. Provide surge tanks between columns with at least 30-60 minutes of

whichever is larger.

them altogether.

holdup each.

1 9

EXISTING COLUMNS-TYPICAL PRACTICES

AN D TROUBLESHOOTING

Although

ths

book deals primarily with &stillation control in design projects,

it is pertinent to consider briefly the controls of typical, existing columns, the

opportunities for their improvement, and how to troubleshoot them when

necessary. Frequently encountered problems include unstable or ineffective con-

trols, off-specification product or products, and f l d g r dumping. In addition,

it is fairly common practice to use excessive boilup and reflux to make sure of

meeting

or exceeding product specifications. This not only wastes energy; it

also reduces column capacity.

To

provide

a

perspective

on

energy savings, one

may note that

100

Ibm/hr of steam is

worth $3200

per year (basis

$4.00/1000

pounds, 8000 hours per year). To save this amount of steam would probably

be only a modest accomplishment for most columns.

If composition control of each product stream is desired (and this is

usually

In some companies

columns

are designed by

the

probabilistic methods recommended

by

Fractionation Research, Incorporated.


19/22

1

IO Cunventwns Followed

in thlj

Book 21

the

case), the most obvious deficiency of most exisring columns is the lack of

appropriate composition measurements. Most commonly temperature in the

upper or lower section of the column (or both) is used

in

lieu of true composition

measurements. Frequently composition control is attempted at only one end

of the column, and sometimes

at

neither end.

Another shortcoming frequently observed is the use of fixed flow controls

for steam, reflux, or product drawoff. Any such unaided flow control should

be regarded with suspicion. With rare exceptions flow-control set points must

be changed to accomplish either composition control or material balance control.

In view of the preceding comments about problem areas and likely op-

portunities for improvement of composition control and reduction in energy

consumption, the following guidelines are suggested:

1.

Make sure that column material-balance controls are properly designed

and tuned, and that hardware, especially level and flow transmitters and control

valves,

is in

good working condition. If

PI

level controllers are’used, follow

the

tuning procedures of Chapter 16; auto overrides or nonlinear controllers

should be used.

It

is usually desirable

to

cascade level control to flow control,

in which case flow measurement should be linear.

2.

Provide averaging level control of column feed. Column feed rate should

also be held between maximum and minimum limits.

3.

Provide a linear flow measurement for steam flow control.

Also

provide

temperature and pressure compensation.

4. If condenser controls are

a

problem, review the control schemes in

Chapter

3.

5.

At this point with flows established, smoothed, and in some cases limited,

it will probably be possible to see some improvement in composition control,

at least for part of the time. For further improvement provide steam-to-feed

ratio control, internal reflux-to-feed or distillate-to-feed ratio control, high

A P

override on steam to protect against flooding, and a minimum steam flow

ltmiter

to

protect against dumping.

6.

If composition is measured and

is

cascade controlled via reflux or boilup,

or both, ratio controls should be replaced by impulse feedforward compensation

see Chapter 12) if feed flow turndown is greater than 2:

1.

7.

For pressurized or vacuum columns, make sure an adequate scheme is

provided for maintaining an inert balance.

8. If temperature

by

itself is not an adequate measure of composition,

consider one of the schemes in Chapter 10 for using temperature, pressure,

and,

in some cases,

flow

measurements

to

deduce compositions.

.

1

I

CONVENTIONS FOLLOWED IN THIS

BOOK

Throughout the remainder of this book, we will frequently illustrate control

schemes with pneumatic components. This is done for convenience. Pneumatics,


20/22

22

Strategy

fm

Dirtillation Column Control

with few exceptions, features a standard signal span of 3- 15psig. LL pneumatic

controllers, as far as we know, can achieve antireset windup by the same external

reset feedback method. This appears

to

be the most universally usell method

(we use it extensively), and has been adopted by one manufacturer of analog

electronic instruments and by several vendors of microprocessor-based distributed

controls. Other vendors of electronic analog and digital controls feature a wide

variety of techniques; some of these work fairly well, while some are quite

inflexible. A brief discussion is presented in Chapter 12.

Units used

in this

book are those commonly employed in chemical engineering:

pounds, feet, degrees Celsius, mols, and

so

on. To facilitate the calculation of

control engineers’ “time constants,” we have mostly used time units of minutes

or seconds [e.g., Ibm/sec, Ibm/min, (pcu/sec>/”C fi’]. For projects that make

partial

use

of metric or SI units,

we

found it convenient

to

convert them

to

the above units. Generally speaking, we have found no advantage in writing

equations with SI units. Instead, in programs for computers or programmable

calculators, we mostly write the equations in the older units and add subroutines

for going back and forth to metric or SI units.

The most common type of controller used in the chemical and petroleum

industries

was

once called “proportional plus automatic reset,” later shortened

to

“proportional reset.” Today it is more common to

use

“PI,” which stands

for proportional-integral. It is also becoming common today to speak of iontroller

gain rather than proportional band

(PB

=

100/Kc).

We

will also

use

“reset

time,” usually in minutes, rather than its older reciprocal, “repeats per minute.”

For drawings in which we are trying to present a perspective of control

concepts (configurations), we

use

very simplified symbols. In drawings where

we are trying to illustrate concepts of structure (overrides, feedfonvard, etc.),

we use a more detailed symbolism that we have found useful in our design

work.

In pneumatics it is c o m o n to refer to most signal-conditioning devices

other than controllers as “relays.” Included are adders, subtractors, multipliers,

and

so

on. For electronic analog and digital controls, it is more common to

use terms such as signal

scalers

and mult@lien.

A table of nomenclature and symbols will be found at the end of this book.

1 .I I LITERATURE

For anyone seriously interested in distillation control, two books are highly

recommended. The first is an easy-to-read, nontheoretic (as far as control is

concerned) work by F. G. Shinskey.” The treatment of energy conservation

alone

is

worth the price of the book. The second book is by Rademaker,

Rijnsdorp, and Maarleveld.12It relies heavily on conventional, single-loop control

theory, and explores painstalungly a large number of possible control systems.

It also contains an extensive bibliography.


21/22

Refevences 3

For basic reference books on distillation, we have made much use of those

Treybal,l6

and

Hengstebeck”

y Van Winkle13 and King.14Others by

also have been

useful.

one by Harriott,” and one bv Murri11.28 For more advanced treatments, we

suggest texts by Koppel,20 Douglas,21 and Gould.22 The last contains some

perceptive comments about

the

difficulties of applving advanced control t heo ry

developed by electrical and mechanical engineers to chemical processes. Recent

books bv Rav”3 and by S te phanopol~ us~~iscuss applications of “modern control

theor .” ’in chemical processes. M c A v o ~ ~as addressed the specific subject of

interaction analysis.

Nonchemical engineers with no background in distillation may find an

introductonr text

bi

Nisenfeld and Seemann

useful.24

t is clearly written and

easv to read.

For basic books on control, we recommend two written by the

1. Buckley,

P.

S., Techniques of Process

Control, Wiley, New York, 1964.

2. Harbert, W. D., Pet. R$27(4): 106-

109

(1948).

3. Harbert, W. D.,

Pet.

Ref 29(10): 117-

122 (1950).

4.Harbert, W. D., Pet. Ref 35(11): 151-

159 (1956).

5. R~ppin,D. W.

T.,

and D. E. Lamb,

“A Theoretical Study of the Dy-

namics and Control of Binan Dis-

tillation Columns,” Bulletin from

the Universitv of Delaware,

1960.

6.

Luyben, W. L., Chem. End.

P q .

61(8): 74-78 (1965).

7. Uitti, K. D., ISA Paper no. 49-9-2.

8. Buckle?,

P.

S., “Override Controls on

a Chemical Reactor,” in Proceedings

of Texas A M Instrumentation

Symposium, Jan. 1970.

9. Buckley,

P. S. ,

“Multivariable Control

in

the Process Industries,” presented

at Purdue University, Apr. 1975.

10. Buckley, P.

S.,

“A

Modem

Perspective

on Controller Tuning,” presented

at Texas A M University, Jan. 17,

1973.

11. Shinskey, F. G., DljtiLLatwn Control,

McGraw-Hill, New York, 1977.

12. Rademaker, O., J.

E.

Rijnsdorp, and

REFERENCES

A. Maarleveld, DynamicJ and Control

of Continuous Distillation Units, El-

sevier, New York, 1975.

13.

Van

Winkle, M.,

DG-tilhim,

McGraw-

Hill. New York, 1967.

14. King, C. J., Separation Processes,

McGraw-Hdl, New York, 1971.

15. Holland, C.

D.,

Fundamentals and

Modelling of Separation Processes,

Prentice-Hall, Englewood Cliffs,

N.J., 1975.

15A. Holland, C. D., and A.

I.

Liapis,

Computer Methodr fm Soltin8 Dy-

u

eparmim

P r o b h

McGraw-

Hill, New York, 1983.

16. Trevbal, R.

E.,

Mms-Tran er Opera-

twns, McGraw-Hill, New York,

1968.

17. Hengstebeck, R. J., Distillation, Rein-

hold, New York,

1961.

18.

Luyben, W.

L.,

ProcessMod.ellin8,Sim-

ulation, and Control f i Chemical

Enginem, McGraw-Hdl, New York,

1973.

19. Harriott,

P.,

Process Control,McGraw-

Hill, New York, 1964.

20. Koppel, L. B., Introduction to Control

T b e o q . . . Prentice-Hd, Engle-

wood Cliffs, N.J., 1968.

21. Douglas, J. M., Process Dynamics and


22/22

24 Stratgy

fm

Dirtillation-Column Control

Control ( 2vols. , Prentice-Hall, En-

glewood Cliffs, N.J., 1972.

22. Gould, L. A., Chemical Process Control,

Addson-Wesley, Reading, Mass.,

1969.

23. Ray,

W.

H.,Advanced Process Control,

McGraw-Hd,

New York, 1981.

24. Nisenfeld, E. I., and R. C. Seemann,

Dictdlation Columns, ISA, Research

Triangle Park, N.C., 1981.

25. Buckley,

P.

S., Materral Balance Control

in Recycle Systems INTECH, May

1974, pp 29-34.

26. McAvoy,

T.,

InteractwnAnalysir In-

strument Society of America,

1983.

27. Stephanopolous,

G.,

Chemical Process

Control, Prentice-Hall, Englewood

Cliffs, N.J., 1983.

28. Murrill,

P.

W., Fundumntals Process

Control Theory, Instrument Society

of America, 1981.

Documents

Distillation Column Control Design